Water heater control system



Jan. 16, 1945.

Filed Jan. 1, 1944 2 Sheets-Sheet 1 i [66' a k E #2 z 1 67 J7 f2 4/ JJ\(rs 1 r 5; 3.5g WM g3 uvmvrox 62 271 Osmmav BY {gig A 7' TORNE Y 219.EULUHUU HU-HHHJ txam n Jan. 16, 1945. c. M. OSTERHELD 2,367,370

WATER HEATER CONTROL SYSTEM Filed Jan. 1, 1944 2 Sheets-Sheet 2 M 6 A389/ /-p/- /3J' INVENTOR.

J OLARk USE/swan A T TORNEY Lib. LLLU i Patented Jan. 16, 1945 UNITEDimmune! WATER HEATER CONTROL SYSTEM Clark M. Osterheld, Stoughton, Wis.,assignor to McGraw Electric Company, Elgin, 111., a corporation ofDelaware Application January 1, 1944, Serial No. 516,692

11 Claims.

My invention relates to electric heating and particularly to tank waterheater control systems.

An object of my invention is to provide a control system for a. domestictank water heater that shall be efiective to prevent energization of thewater heater when another circuit in the same house or home is carryinga heavy load.

Another object of my invention is to provide a control system for a.domestic tank water heater that shall be effective to cause energizationof a water heater with a preset time period of delay after a secondcircuit has had its load current reduced below a predetermined value incase only a, relatively small amount of hot water has been withdrawnfrom the tank and that shall cause immediate energization of the waterheater after said second circuit has had its load current reduced belowa predetermined value in case a relatively large amount of hot water hasbeen withdrawn from the tank.

Other objects of my invention will either be apparent from a descriptionthereof or will be set forth during the course of such description andpointed out particularly in the appended claims.

In the drawings,

Figure l is a vertical sectional view of a domestic hot water tank witha thermal retarder unit at the center and a heating element at the lowerportion of the tank,

Fig. 2 is a diagram of the electric circuits for the heating elementretarder contacts and peak load limiter in connection with the houselighting, range and washing machine load,

Fig. 3 is a top plan view of the thermal retarder shown in Fig. 5,

Fig. 4 is a. horizontal sectional view therethrough taken on the line 44of Fig. 5,

Fig. 5 is a vertical sectional view on the'line 5-5 of Fig. 4 showingthe positions of the elemen s of the thermal retarder when there is coldwater in the center of the tank, and,

F 6 is a vertical plan view of the retarder showin its elements in theirpositions when the water in the center of the tank is hot.

Referring first of all to Fig. 1 of the drawings, I hav there shown ahot water tank ll having a lower cold water inlet pipe I3 and an upperhot water outlet pipe l5 mounted thereon. I provide also a covering llof suitable heat-insulating material, such as rock wool, and an outercasing 19 all as now well known in the art.

While I have shown a single electric heater 2| mounted thereon andlocated in a tunnel 23, I

do not desire to be limited thereto. If only a single heater is used, Ilocate it adjacent the lower end of the tank.

I have shown a first thermal heater control switch designated generallyby the numeral 25 and have illustrated this schematically as comprisinga tube 21 having a closed inner end and having its outer end secured ina. fluid-tight manner in an opening in the wall of the tank H near thelower end thereof. I provide an expansion rod 29 in the tube 21 adaptedto engage a lug 3| oi electric-insulating material which lug is securedto the outer end of a resilient contact arm 33 having a contact memberthereon adapted to engage with and be disengaged from an innersubstantially rigid contact arm 35. I have shown supporting and mountingmeans for the two contact arms 33 and 35 as comprising blocks 31 ofsuitable electric-insulating material.

Referring now to Fig. 2 of the drawings I have there shown a diagram ofconnections of the control system embodying my invention which includesin addition to the main or lower thermally-actuable heater controlswitch 25 a thermal retarder heater control switch unit 39 to behereinafter described in detail.

I have shown a pair of supply circuit conductors 4| and 43, supplycircuit conductor 43 bein connected by a. conductor 45 with resilientcontact arm 33.

I have shown an electromagnetic heater control switch designated in itsentirety by numeral 41, this switch including a pair of substantiallyfixed contact members 49 and 5| a contact bridging member 53 secured toa magnetic core 55 adapted to be energized by current flowing through a,coil 51.

The electromagnetic switch 41 may be of the usual contactor type and sodesigned that upon the coil 51 being traversed by a current greater thana predetermined value, contact bridging member 53 will be moved upwardlyout of engagement with contact members 49 and 5|.

The heater circuit including the heater 2| is the first house loadcircuit and I have shown a second house load circuit adapted to haveconnected thereto and therewith the lights 59 in an ordinary home, anelectric range 6| as well as the motor of a washing machine 63. Members59, BI and 63 are shown schematically only, each of these beingcontrolled by the usual or suitable switches. One terminal of themembers 59. 3| and 83 may be connected to a conductor 65 65 which isconnected to supply circuit conductor 43. The other terminal of theelements 59, BI and 63 may be connected to a second conductor 61 whichis connected to one terminal of coil 51, the other term nal of coil 51being connected to supply circuit conductor 4|. Contact member 49 isconnected by a conductor 69 to one terminal ll of the thermal retarderswitch unit while the other terminal 13 thereof is connected by aconductor 15 to one terminal of heater 2|. The other terminal of heater2| is connected first to rigid contact arm 35 while it is also connectedby a conductor 11 to one term nal of a small heating coil I9constituting a part of the thermal retarder switch unit to behereinafter described in detail. The other terminal of heating coil 19is connected by a conductor 8| with conductor 69 hereinbefore described.

Referring now to Figs. 3 to 6 inclusive. I have there illustrated oneform of thermal retarder switch urit 39 which as shown in Fig. l of thedrawings may be mounted in heat-conducting relation on the outsidesurface of the tank I I at substantially the mid-portion thereof. Iprovide an opening 83 in the mass I1 of heat insulation to receive thethermal retarder unit, and I may provide also a cover 85 for saidopening.

The thermal retarder heater control switch unit includes a firstthermally-expansible rod 81 which has a lower end portion 89 of reduceddiameter adapted to fit into an opening in a block I 9| ofheat-conducting material. The block 9| is provided with a pair oflaterally-extending ears or lugs 93, through which extend a pair ofmachine screws 95 securing block 9| against a block 91 which has anarcuate inner surface and which may be held in proper operative positionagainst the outer surface of tank II as by welding seams 99. Block 91 isalso made of heat-conducting ma terial such as a suitable metal oralloy.

I provide a block of heat-insulating material which may be held againstthe front surface of block 9| as by a pair of machine screws I03. BlockIN is provided with an aperture therethrough into which fits the lowerreduced end portion of a second expansion rod I having a central boreI01 extending therein from one end thereof. I provide a support |09 forthe small heating coil 19, the terminals of heating. coil 19 beingconnected to terminal members H I. An outer casing ||2 surrounds thesecond expansion rod I05 and is of heat-insulating material.

The upper reduced end portion of the first high expansion rod 8'! has ashort lug II3 secured thereto while the upper reduced end portion of thesecond expansion rod I05 has secured thereto a short lug H5. The twoarms extend toward each other. It may be here pointed out that while thefirst expansion rod 81 is of a relatively highly expansive material suchas aluminum, the second expansion rod I05 is of a material such as metalor alloy having a lesser thermal expansivity than rod 81. I may useInvar for the second rod I 05 or some other nickel steel or a chromesteel. The thermal expansivity of the second rod I05 may be on the orderof one-half that of the first expansion rod 81.

It i to be noted that the ordinary temperature of cold water is 65 or 70F. and that hot water has a temperature of 150 F. or slightly higher.Thus the first high expansion rod 81 will have its maximum length whensubject to a temperature of 150 F. while the second low expansion rodI05 will have its maximum length when its temperature is on the order of300 F. or higher.

At these respective high temperatures $25 pansion characteristic of thetwo rods will be substantially the same, that is the lengths of the tworods will be substantially equal, and the switch controlled thereby willbe closed.

A heater control switch actuable by the relative expansion of the tworods 81 and I05 may include an upper double bar toggle arm II! which isof substantially L-shape, the horizontally extending portion thereofbeing pivotally supported on pivot pins H9 and |2| extending through thetwo arms and into the end portions of lugs H3 and I I5. A cross bar |23may be provided to hold the two bars of toggle arm II in properoperative position relatively to each other. I provide a lower or secondtoggle arm I25, the upper end of which has pivotal engagement with thelower end portion of the bars of toggle arm Ill, and an over centerspring |2'| has its upper end connected to a cross bar I29 extendingthrough the bars of toggle arm I|'| while the lower end of spring I2! isconnected with an intermediate portion of arm I25.

I provide a block I3I mounted on the lower end of toggle arm I25, thisblock supporting a contact bridging member I33 which is insulatedlymounted on block |3|. Bridging member I33 is adapted to engage with andbe disengaged from the fixed contact members II and 13 hereinbeforedescribed.

I provide further a bracket I35 which may be suitably secured againstthe lower surface of block IM and which has an adjustable stop screw I31thereon. The bracket I35 may also be utilized to support terminals I IIwhich may be mounted on a block I39 of electric-insulating materialsupported by bracket I35.

The thermal retarder heater control switch unit is disclosed and claimedin my co-pending application of United States Letters Patent S. N.511,387, filed November 22, 1943, and assigned to the same assignee asis the present application.

As has been hereinbefore stated one of the main objects of my inventionis to ensure that the heater of the hot water tank shall not beenergized during those times of the day or night when the second circuitenergizing electric motors, the range, or the lights of the usual homeis traversed by a relatively heavy current. In other words one of theobjects of my invention is to prevent the occurrence of relatively heavyoverloads or even relatively heavy loads on the supply circuit connectedwith the ordinary home. This is effected by the operation of theelectromagnetic switch 41 which, when its coil is traversed by more thana predetermined amount of current, will interrupt the circuit to theheater 2| for the tank.

Another object of my invention is to provide a control system that shallbe operative to cause immediate energization of the heater for a hotwater tank after decrease of current in the second house-load circuitbelow a predetermined amount and that shall cause energization of theheater of the tank with a predetermined time delay in case only arelatively small amount of hot water has been withdrawn from the tank.It is to be understood that the heater 2| is to be energized immediatelyupon reclosure of the switch 4'! controlled by the current in the secondload circuit if the thermal retarder is subject to cold water in thetank, that is so much hot water has been withdrawn that substantiallyless than onehalf of the water in the tank is hot, this hot water beinglocated in the upper portion of the tank. Delayed energization of theheater 2| is to be effected in case the lower thermal heater controlswitch 25 only i subject to cold water.

The thermal retarder switch unit is shown in Fig. 5 or the drawings inits closed circuit position which will occur if the temperature of thetwo expansion rods 81 and I05 are at substantially the same value. Thiswill be the case when the tank I I is first filled with cold water,under which conditions the lower thermal heater control switch will beclosed andthe two expansion rods 81 and I05 will be at substantiallyordinary room temperature. If it be assumed that the load connected withthe second circuit is small enough so that switch 41 is closed, theauxiliary heater coil I9 will be energized, but the design, constructionand adjustment of the thermal retarder is such, preferably, that it willrequire from three to five hours before the second expansion rod I05will have been heated sufficiently to cause expansion thereof to anamount to cause operation of the switch to its closed circuit position.If the lengths of rods 81 and I05 are short, the switch will be inclosed circuit position but if the first and high expansion rod 81 issubject to the temperature of hot water in the tank, while the heatingcoil I9 has been energized less than a predetermined length of time, theswitch will be in the open circuit position substantially as shown inFig. 6 of the drawings.

The thermal retarder switch will be in the open circuit position whensuflicient water in the upper portion of the tank has been heated sothat the bottom dividing line thereof is below the thermal retarderswitch 39. If this amount of water was heated in less time than requiredfor the second expansion rod I05 to be heated to its maximum operatingtemperature, on the order of 300 F. or over, the switch will be moved toopen position, but will again be moved to closed position when thesecond rod I05 has been heated sufficiently by reason of the continuedenergizetion of heating coil I9.

Energization of the heater 2| will continue until substantially all ofthe water in the tank has been heated to a temperature on the order of150 F., at which time the lower thermal heater control switch 25 will beeffective to open the circuit for the heater 2|.

If now during a part of the day when the current in the second loadcircuit is below said predetermined value, hot water is withdrawn fromthe tank in a relatively small quantity only, suflicient to subject thetube 21 to cold water, the closed switch 25 and the closed switch 41will cause energization of the heating coil I9 that the thermal retarderswitch will be moved to closed position with an adjustable predeterminedtime delay period because of the expansion of rod I05. If, however, arelatively large amount of hot water is withdrawn, during this delayperiod, so that the thermal retarder switch is subject to cold water,the contraction of the first expansion rod 81 will be such as to causeimmediate closure of the first circuit including the heater It isobvious that the heater control system embodying my invention andincluding a form of thermal retarder either that shown in the drawingsor its equivalent will be effective to prevent energization of the tankwater heater during such times of a twenty-four hour day when the loadon the other circuits of the house is above a predetermined amount, willpermit immediate energization of the tank water heater when relativelylarge amounts of hot water have been withdrawn from the tank and willcause energization of the tank water heater with a predetermined timeperiod of delay in case only relatively small amounts of hot water havebeen withdrawn from the tank, during such time as the load on the otherhouse circuits is below said predetermined amount.

Various modifications may be made in the system embodying my inventionwithout departing from the spirit and scope thereof and all suchmodifications clearly coming within the scope of the appended claimsshall be considered to be covered thereby.

I claim as my invention:

1. A water heater control system for a hot water tank having an electricheater, comprising a first electric circuit for said heater, a heatercontrol switch in said circuit, a second electric circuit, switchingmeans for said electric heater in said first circuit controlled by thecurrent flowing in said second circuit and moved to open posi-- tionwhen more than a predetermined amount of current is flowing in saidsecond circuit and a thermal retarder for actuating said heater controlswitch, said thermal retarder comprising a first thermally-expansivemember subject to tank water temperature at an intermediate portion ofthe tank a second thermally-expansive member out of close thermalrelation with said tank, a heating coil for said second membercontrolled by said switching means and means mechanically connectingsaid two thermally-expansive members to said heater control switch tocause closur thereof when said two thermally-responsive members are atsubstantially ordinary room temperature and attendant energization ofsaid electric heater immediately after reclosing of said switching meansupon reduction of the amount of current flowing in said second circuitto a value below said predetermined amount and in case the firstthermally-responsive member is subject to cold water.

2. A water heater control system for a hot water tank having an electricheater, comprising a first electric circuit for said heater, a heatercontrol switch in said circuit, a second electric circuit, switchingmeans for said electric heater in said first circuit controlled by thecurrent flowing in said second circuit and moved to open position whenmore than a predetermined amount of current is flowing in said secondcircuit and a thermal retarder for actuating said heater control switch,said thermal retarder comprising a first thermally-expansive membersubject to tank water temperature at an intermediate portion of the tanka second thermally expansive member out of close thermal relation withsaid tank, a heating coil for said second member controlled by saidswitching means and means mechanically connecting said twothermally-expansive members to said heater control switch to causeopening thereof when the expansion characteristics of said twothermally-responsive members are substantially the same and to causeclosure of said heater control switch and attendant energization of saidheater after a predetermined time period of delay after reclosing ofsaid switching means upon reduction of the amount of current flowing insaid second circuit below said predetermined amount, in case the firstthermallyresponsive member is subject to hot water.

3. A water heater control system for a hot water tank having an electricheater, comprising a first electric circuit for said heater, a heatercontrol switch in said circuit, a second electric circuit, switchingmeans for said electric heater in said first circuit controlled by thecurrent flowing in said second circuit and moved to open position whenmore than a predetermined amount of current is flowing in said secondcircuit and a thermal retarder for actuating said heater control switch,said thermal retarder comprising a first thermally-expansive membersubject to tank water temperature at an intermediate portion of the tanka second thermally-expansive member out of close thermal relation withsaid tank, a heating coil for said second member controlled by saidswitching means and means mechanically connecting said twothermally-expansive members to said heater control switch to selectivelyactuate the latter to open and to closed position in accordance with therelative temperatures of the two thermally-responsive members andselectively effective to cause energization of the heater substantiallyimmediately after reclosure of said switching means upon reduction ofthe amount of current flowing in said second circuit below saidpredetermined amount in case said first thermally-responsive member issubject to cold water, to delay energization of the heater for apredetermined time period of delay after reclosure of said switchingmeans in case said first thermallyresponsive member is subject to hotwater and to cause energization of said heater during said delay periodin case said first thermally-responsive member was subject to hot waterat the time of reclosure of said switching means and then became subjectto cold water during said delay period because of the withdrawal of hotwater from the tank.

4. A water heater control system for a hot water tank having an electricheater, comprising a first electric circuit for said heater, a firstheater control switch in said circuit, a second electric circuit,switching means for said electric heater in said first circuitcontrolled by the current flowing in said second circuit and moved toopen position when more than a predetermined amount of current fiows insaid second circuit, a thermal heater control switch in said firstelectric circuit subject to tank water temperature at the lower endportion of the tank, said first heater control switch, said switchingmeans and said thermal heater control switch being connected in serieswith each other and with the electric heater and a thermal retarder foractuating said first heater control switch, said thermal retardercomprising a first thermally-expansive member subject to tank watertemperature at an intermediate portion of the tank, a secondthermally-expansive member out of close thermal relation with said tank,a heating coil for said second member controlled jointly by saidswitching means and said thermal heater control switch and meansmechanically connecting said two thermally-expansive members to saidilrst heater control switch to cause closure therec when said twothermallyexpansive members are at substantially the same relatively lowtemperature with attendant energization of said heater immediately afterreclosing of said switching means upon reduction of the amount ofcurrent flowing in said second circuit to a value below saidpredetermined amount and in case said first thermally-responsive memberis subject to cold water.

5. A water heater control system for a hot water tank having an electricheater, comprising a first electric circuit for said heater, a firstheater control switch in said circuit, a second electric circuit,switching means for said electric heater in said first circuitcontrolled by the current flowing in said second circuit and moved toopen position when more than a predetermined amount of current flows insaid second circuit, a thermally-actuable heater control switch in saidfirst electric circuit subject to tank water temperature at the lowerend portion of the tank, said first heater control switch, saidswitching means and said thermally-actuable heater control means beingconnected in series with each other and with the electric heater and athermal retarder for actuating said first heater control switch, saidthermal retarder comprising a first thermally-expansive member subjectto tank water temperature at an intermediate portion of the tank, asecond thermally-expansive member out of close thermal relation withsaid tank, a heating coil for said second member controlled jointly bysaid switching means and said thermally-actuable heater control switchand means mechanically connecting said two thermally-expansive membersto said first heater control switch to cause closure thereof when saidtwo thermally-expansive members are 'at substantially the samerelatively low temperature with attendant energization of said heaterimmediately after reclosing of said switching means upon reduction ofthe amount of current flowing in said second circuit to a value belowsaid predetermined amount and in case said first thermallyresponsivemember is subject to cold water, and to cause closure of said firstheater control switch with a predetermined time period of delay afterreclosure of said switching means upon reduction of the amount ofcurrent flowing in said second circuit to a value below saidpredetermined amount and in case said first thermally-responsive memberis subject to hot water and said thermally-actuable heater controlswitch is subject to cold water.

6. A water heater control system for a hot water tank having an electricheater, comprising a first electric circuit for said heater, a firstheater control switch in said circuit, a second electric circuit,switching means for said electric heater in said first circuitcontrolled by the current flowing in said second circuit and moved toopen position when more than a predetermined amount of current fiows insaid second circuit, a thermally-actuable heater control switch in saidfirst electric circuit subject to tank water temperature at the lowerend portion of the tank, said first heater control switch, saidswitching means and said thermal heater control means being connected inseries with each other and with the electric heater and a thermalretarder for actuating said first heater control switch, said thermalretarder comprising a first thermally-expansive member subject to tankwater temperature at an intermediate portion of the tank, a secondthermally-expansive member out of close thermal relation with said tank,a heating coil for said second member controlled jointly by saidswitching means and said thermal heater control switch and meansmechanically connecting said two thermally-expansive members to saidfirst heater control switch to cause closure thereof when said twothermallyexpansive members are at substantially the same relatively lowtemperature with attendant energization of said heater immediately afterreclosing of said switching means upon reduction of the amount ofcurrent flowing in said second circuit to a value below saidpredetermined amount and in case said first thermally-responsive memberis subject to cold water, and said thermally-actuable heater controlswitch being efiective to cause deenergization of said electric heaterwhen substantially all of the water in the tank is hot.

7 A water heater control system for a hot water tank having an electricheater, comprising a first electric circuit for said heater, a firstheater control switch in said circuit, a second electric circuit,switching means for said electric heater in said first circuitcontrolled by the current flowing in said second circuit and moved toopen position when more than a predetermined amount of current flows insaid second circuit, a thermally-actuable heater control switch in saidfirst electric circuit subject to tank water temperature at the lowerend portion of the tank, said first heater control switch, saidswitching means and said thermal heater control means being connected inseries with each other and with the electric heater and a thermalretarder for actuating said first heater control switch, said thermalretarder comprising a first thermally-expansive member subject to tankwater temperature at an intermediate portion of the tank, a secondthermally-expansive member out of close thermal relation with said tank,a heating coil for said second member controlled jointly by saidswitching means and said thermallyactuable heater control switch andmeans mechanically connecting said two thermally-expansive members tosaid first heater control switch to cause closure thereof when said twothermally-expansive members are at substantially the same relatively lowtemperatur with attendant energization of said heater immediately afterreclosing of said switching means upon reduction of the amount ofcurrent flowing in said second circuit to a value below saidpredetermined amount and in case said first thermally responsive memberis subject to cold water, and to cause closure of said first heatercontrol switch with a predetermined time period of delay after reclosureof said switching means upon reduction of the amount of current flowingin said second circuit to a value below said predetermined amount and incase said first thermallyresponsive member is subject to hot water and8. A water heater control system for a hot water tank having an electricheater, comprising a first electric circuit for said heater, a heatercontrol switch in said circuit, a second electric circuit, switchingmeans for said electric heater in said first circuit controlled by thecurrent flowing in said second circuit and moved to open position whenmore than a predetermined amount of current is flOWiIlg in said secondcircuit, a thermally-actuable heater control switch connected in seriescircuit with said heater control switch and switching means and athermal retarder for actuating said heater control switch, said thermalretarder comprising a first thermally-expansive member subject to tankwater temperature at an intermediate portion of the tank, a secondthermally-expansive member out of close thermal relation with said tank,a heating coil for said second member controlled by said switching meansand means mechanically examiner connecting said two thermally-expansivemembers to said heater control switch to cause opening thereof when saidtwo thermally-responsive members are at substantially differenttemperatures and to cause closure of said heater control switch andattendant energization of said heater after a predetermined time periodof delay after reclcsing of said switching means upon reduction of theamount of current flowing in said second circuit below saidpredetermined amount in case the first thermally-responsive member issubject to hot water and said thermally-actuable heater control switchbeing effective to cause deenergization of said electric heater whensubstantially all of the water in the tank is hot.

9. A control system for an electric heater for a hot water tankcomprising a heater circuit, a second house load circuit, threedependent switch units for opening and closing said heater circuit, thefirst switch unit being controlled by the current in said second circuitand being in closed position when the current in said second circuit isbelow a predetermined amount, the second switch unit being athermally-actuable switch subject to tank water temperature at the lowerend of the tank and being closed when subject to cold water, the thirdswitch unit including a thermo-sensitive unit subject to heat exchangewith tank water intermediate the ends of the tank and anotherthermo-sensitive element subject to heat exchange means independent ofthe tank water and operated by power from the circuit when said firstand second switches are closed, said switches being effective toselectively energize said heater immediately and with a predeterminedtime delay period after closure of said first switch in dependence uponthe amount of cold water then in the tank.

10. A control system for an electric heater for a hot water tank,comprising a heater circuit, a second house load circuit, three switchunits connected in series circuit with each other and the electricheater for opening and closing said heater circuit, the first switchbeing controlled by the current in said second circuit and being inclosed position when the current in said second circuit is below apredetermined value, the second switch unit being a thermally-actuableswitch subject to tank water temperature at the lower end of the tankand being closed when subject to cold water, the third switch being athermal retarder switch including one thermo-sensitive element subjectto heat exchange with tank water intermediate the ends of the tank, asecond thermo-sensitive element subject to heating applied independentlyof tank water and heating means operated by power from the circuit fcrheating the second element when said first and second switches areclosed, said switches being jointly effective to cause energization ofsaid electric heater with a predetermined time delay pericd afterclosure of said first switch in case the tank contains only enough coldwater to reach said second switch and immediately after closure of saidfirst switch in case the tank contains enough cold water to effect saidone thermo-sensitive element of the third switch.

11. A control system for an electric heater for a hot water tankcomprising a heater circuit, a second house load circuit, threedependent switch units for opening and closing said heater circuit, thefirst switch unit being controlled by the current in said second circuitand being in closed position when the current in said second circuit isbelow a predetermined amount, the second switch unit being athermally-actuable switch subject to tank water temperature at the lowerend of the tank and being closed when subject to cold water, the thirdswitch unit including a thermo-sensitive unit subject to heat exchangewith tank water intermediate the ends of the tank and anotherthermo-sensitive element subject to heat exchange means independent 01'the tank water and operated by power from the circuit when said 10 firstand second switches are closed, said switches being eflective toselectively energize said heater immediately and with a predeterminedtime delay period after closure of said first switch in dependence uponthe amount of cold water then in the tank, said second switch beingeffective to deenergize said electric heater when substantially all ofthe water in the tank is hot.

CLARK M. OS'I'ERHEID.

